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Abstract

The visual system’s ability to adjust its contrast sensitivity to the ambient contrast level is known as gain control. Previous research has shown that the adjustment of contrast sensitivity is driven by stimulus parameters in a bottom-up fashion [1] and further is under attentional control [2]. Here we present evidence that contrast sensitivity is under top-down control. We
paired an absolute identification paradigm with a cueing paradigm to measure identification accuracy for four low contrast stimuli that were intermixed with an occasional high contrast stimulus. Foreknowledge of the presentation of the high contrast stimulus was manipulated by
presenting valid and invalid-cues. In Experiment I we found that identification and discriminability of low contrast stimuli was impaired only when the occurrence of the high contrast stimulus was unpredictable but not when it was predictable. Only a top-down gain control
mechanism whose functions were to maximize discriminability while protecting the system
from overload was consistent with the results. Experiment II provided further support for the idea that an important function of gain control is to protect visual mechanisms from overload. A fit of the Naka-Rushton contrast response model to our data suggested that the top-down control mechanism reduced the effective contrast of the visual pattern (contrast gain) and rescaled the response of contrast sensitive units (response gain).